Polyurethanes are widely known and often involve the initial preparation of a prepolymer formed by the reaction of an aliphatic or aromatic polyisocyanate with a long chain polyol. The resulting prepolymer formed by such reaction can then be reacted with a chain extending agent such as water, a diamine or short chain diol to form the final polyurethane polymer. The polyurethanes formed by this basic reaction have multiple applications, such as adhesives, elastomers, coatings, and sealants. Properties of the resulting polyurethane are controlled by the proper selection of isocyanate, long chain polyol and chain extending agent. Each application requires different processing parameters such as appropriate pot life, viscosity, etc. Each application also has its own specific requirements for end use properties, for example: elastomers may require excellent tensile strength, elongation, tear strength, etc.; adhesives may require excellent tack, peel and shear strength; and flexible and nonflexible coatings may require excellent solvent resistance, scratch resistance and abrasion resistance. Many of these requirements may be met by proper design of the prepolymer.
Polyurethane systems for coatings are formulated to give good solvent resistance, tensile properties and scratch resistance. Polyurethane coatings are also formulated for good flexibility coupled with toughness, excellent electrical properties and good abrasion resistance. However, the prepolymers which formed the basis of such systems were generally of relatively high molecular weight and gave coating solutions of relatively low non-volatiles content (&lt;50% by weight).
Due to recent environmental concerns about solvent emissions there is pressure to develop formulations with higher non-volatile content but which retain good physical properties. The general approach has been to lower the molecular weight of the polyurethane prepolymers, thus lowering the viscosity of the resin and allowing an increase in the non-volatiles of the formulation. Such formulations could be cured into high molecular weight polymers by various mechanisms known in the art such as curing with atmospheric moisture or with a second component.
Representative patents which show various polyurethane prepolymers and various end products produced by reaction of a diamine or a diol with the resulting polyurethane prepolymers are as follows:
British Patent 1,101,410 discloses polyurethane prepolymer elastomers having low viscosity and extended pot life which are prepared by reacting a diisocyanate with a diol wherein the diisocyanate is present in a mole ratio of 1:1 and preferably 1.1:1.7. Although up to 7 moles diisocyanate per mole of diol can be used, its use, at that level, is discouraged because any excess diisocyanate used in preparing the polyurethane prepolymer must be removed in order to control final properties.
U.S. Pat. No. 3,218,348 discloses a process for preparing polyurethane polyisocyanates which have high molecular weight and do not crystallize from a solution on standing. The polyisocyanates are reacted with a trihydric alcohol such as trimethylolpropane in an organic solvent followed by addition of a dihydric alcohol such as 1,3-butyleneglycol.
U.S. Pat. No. 3,384,624 discloses a process for preparing polyurethane prepolymers free of unreacted polyisocyanate. The prepolymers, which can be used for preparing coatings, castings, paints and lacquers, are prepared by reacting toluenediisocyanate with an active hydrogen containing compound, e.g. a long chain diol and then contacting the prepolymer with a phenolic material in an amount sufficient to remove excess unreacted polyisocyanate. Mole ratios of polyisocyanate to diol range from about 1.3 to 2.1. The resulting blocked polyurethane prepolymer can then be unblocked and chain extended with an organic diamine or polyol.
U.S. Pat. No. 3,726,825 discloses polyurethane coatings having moisture vapor barrier properties as well as high gloss, abrasion resistance, etc. required of such coatings. The linear thermoplastic polyurethane resins are prepared by reacting a non-halogenated organic diisocyanate with an organic dihydroxy compound and from about 0.1 to 0.9 moles of neopentylglycol. The resulting polyurethane prepolymer then is cured under anhydrous conditions.
U.S. Pat. No. 4,786,703 discloses various polyurethane prepolymers suited for producing polyurethane/urea elastomers, the prepolymers being formed by reacting an excess of toluenediisocyanate with a long chain diol. Unreacted toluenediisocyanate then is stripped from the resulting prepolymer. Utilization of high mole ratios, e.g. 4-12:1 toluenediisocyanate per mole of diol results in a prepolymer with very low content of higher oligomers. When the prepolymer is cured with an aromatic diamine, property enhancements of the resulting polyurethane elastomer are achieved.
U.S. Pat. No. 4,182,825 and U.S. Pat. No. 4,888,442 disclose the removal of unreacted isocyanate monomers from polyurethane prepolymers to enhance properties. Low hysteresis is reported. The disadvantage of such prepolymers is that the removal of the polyisocyanate from a conventional prepolymer increases the viscosity of the prepolymer. As a result the coating must be formulated at lower non-volatiles content.